Stylus support assembly with flexed ligament hinge

ABSTRACT

A stylus support assembly for a gauge for a metrological instrument which has a main gauge body and a mounting block for supporting a stylus. The main gauge body and the mounting block each have first and second ends, with the first ends facing in the same direction. A resilient ligament connects the first ends which are arranged at an angle to one another so that the ligament biases the mounting block and thus the stylus to extend at an angle to the main gauge body, thereby providing a biasing force to bias the stylus against the workpiece.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a stylus support assembly for a gaugefor a metrological instrument for measuring surface form such asroundness and/or surface finish parameters, such as surface texture orroughness.

2. Discussion of the Background

Metrological instruments are known in which a stylus is mounted on anarm so as to follow the surface form or finish of a workpiece duringrelative movement of the arm and the workpiece. A transducer outputs asignal in accordance with movement of the stylus relative to the arm.Metrological instruments are normally made so that the stylus isremovable and replaceable. This is to allow a different stylus to befitted to the same machine. This may be necessary to allow the styluslength to be changed or to change the particular stylus tip, inaccordance with the workpiece being measured and the accuracy required.However, the constant insertion and removal of different styli meansthat the gauge must be constantly recalibrated and leads to inaccuratereadings from the transducer if recalibration is not carried out.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided astylus support assembly which supports a stylus in a more stable andreliable manner.

According to one aspect of the present invention, there is furtherprovided a stylus support assembly for supporting a stylus for use in agauge for a metrological instrument, the support assembly comprising twomembers resiliently connected so as to provide a stylus force in use ofthe assembly to enable the stylus tip to be pressed against the surfaceof the body to be measured and keep contact with that surface.

The members may be connected at an angle which is typically within arange of 2° to 3° from the perpendicular to the gauge axis. The stylusforce can be changed by changing the resilience and/or the angle of theconnecting means.

In another aspect, the present invention provides a one-piece stylus. Astylus tip can be mounted at the front end. A shaft for holding a coreof a transducer ferrite, can be located at the rear end of the stylus.

In one aspect, the invention provides a particular construction ofstylus support means to improve stylus stability.

A stylus support assembly embodying the invention is easy to manufactureand assemble.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention, given by way of non-limiting example, willnow be described with reference to the accompanying drawings in which:

FIG. 1 is a schematic view of a metrological instrument embodying theinvention;

FIG. 2 shows a part-sectional view of a stylus support assemblyaccording to the present invention;

FIG. 3 shows the front end of the stylus support assembly shown in FIG.2;

FIG. 4 shows a plan view of the stylus support assembly;

FIG. 5 shows an align met block, used to fit a stylus tip in the stylussupport assembly shown in FIG. 3; and

FIG. 6 shows an exaggerated enlarged view of part of the stylus shown inFIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic view of a metrological instrument. A workpiece 10is placed on a turntable 16. A gauge body 13 is mounted in conventionalmanner (for example by means of a carriage (not shown)) to a fixedcolumn 11 so as to enable the gauge body to move transversely of thecolumn, that is toward or away from the workpiece. The gauge body 13 mayalso be movable up and down the column to allow, for example,measurements to be made at different heights above the turntable 16. Thegauge body which may have a diameter of, typically, 15 mm provides astylus/support assembly to which a stylus 3 having a stylus tip 12 ispivotably mounted. In use, the gauge body is moved horizontally (that istoward the workpiece) to bring the stylus tip 12 into contact with thesurface of the workpiece. Variations in the surface form and/or finishof the workpiece cause the stylus to pivot. This pivoting movement isdetected in known manner by a linear variable differential transducer(LVDT) located within the gauge body 3. The output signal of thetransducer is supplied to processing means (not shown), such as apersonal computer, which obtains from the transducer output signalinformation about the surface form and/or finish of the workpiece.

FIGS. 2 and 3 illustrate in greater detail a stylus support assemblyembodying the invention. The stylus support assembly comprises anelongate first member or main gauge body 1 which is typically formed ofstainless steel and a second member or mounting block 2 which has athrough aperture (not shown) through which the stylus 3 extends. Thestylus 3 is held fixedly within the mounting block 2 by means of, forexample, adhesive.

The mounting block 2 is typically formed of stainless steel and isgenerally cylindrical in shape and has an upper face 2 a which isreceived with clearance in a recess 1 a formed by a smaller diametersection of the main gauge body adjacent a front end face 1 b thereof.

The front end face 1 b of the main gauge body 1 is formed so as not tobe at 90° to the longitudinal axis of the main gauge body but rather tobe at an angle of, typically, approximately 87 to 88°, in this example87.5°, thereto so that the portion of the end face 1 b remote from themounting block 2 projects beyond. the mounting block. The angling of thefront end face 1 b of the main gauge body may be achieved by, forexample, conventional grinding and lapping processes.

The mounting block 2 is resiliently connected to the main gauge body bymeans of a resilient connecting member or ligament 4 which is fixedlysecured to the front end face 1 b of the main gauge body 1 and to themounting block 2 by means of a two-piece ligament clamp 5, as shown inFIG. 4.

As can most clearly be seen from FIG. 3, the ligament 4 is a plate-likemember in the form of a major segment of a circle having a recess 4 afor accommodating the stylus 3 while the two-piece clamp is provided bya first generally semi-circular clamping member 5 a and a secondclamping member 5 b which differs in shape from the first clampingmember 5 a by being modified to fit the shape of the lower section ofthe ligament. The first clamping member 5 a is secured to the end face 1b of the main gauge body 1 by means of screws 9 (three are shown in FIG.3) which extend through holes in the first clamping member 5 a and theligament 4. The second clamping member 5 b is similarly secured to anend face 2 b of the mounting block 2 by screws 9 (two are shown in FIG.3).

The ligament 4 may be formed of, for example, beryllium copper while theclamping members 5 a and 5 b may be manufactured from stainless steelwith a thickness of approximately 1.4 mm.

As can be seen from FIGS. 2 and 3, the ligament 4 extends acrosssubstantially the entire width of the end face 1 b of the main gaugebody 1 and across the width of the mounting block 2. The fact that theligament 4 extends across the full width of the main gauge body shouldgive greater stability for the stylus support assembly making twistingof the stylus less likely

The mounting of the mounting block 2 to the main body 1 by the ligament4 provides a small clearance between the mounting block and the recess 1a of the main gauge body across which, as can be seen from FIGS. 2 and3, only the ligament 4 extends. This arrangement allows the mountingblock 2, and thus the stylus 3 carried by the mounting block 2, to pivotrelative to the main body about the free area of the ligament 4.

The angling of the end face 1 b of the main body causes the ligament 4to bias the front end 3 a of the stylus 3 away from the main gauge bodyin FIG. 2 so as to provide a stylus force biassing the stylus tipdownwardly in FIG. 2. As will be appreciated, when the gauge shown inFIG. 2 is mounted in the instrument shown schematically in FIG. 1, thegauge will be rotated through 90 about its longitudinal axis from theorientation shown in FIG. 2 so that the ligament 4 biasses the stylus ina direction out of the paper in FIG. 1, that is towards the workpiece10.

This arrangement for providing the stylus force is relatively simple toassemble in contrast to the conventional spring biasing arrangementsused in, for example, the Talyrond 300 manufactured by Taylor HobsonLimited, Leicester, England which can be extremely difficult tocalibrate and to assemble within the gauge.

The stylus force provided by the ligament 4 may be adjusted by changingthe thickness of the ligament or by selecting a different material witha different resilience.

It should be appreciated that the biasing force results from the factthat the end face 1 b of the main gauge body 1 is at an angle to the endface 2 b of the mounting block 2 and that a similar effect could beachieved by angling the end face 2 b of the mounting block 2 rather thanthe end face 1 b of the main gauge body 1 so that the end face 2 b ofthe mounting block 2 and the end face 1 a of the main gauge body 1subtend an angle of less than 180° typically 177-178°.

Although the stylus 3 carried by the mounting block may be aconventional stylus, in this example the stylus is in the form of aone-piece body which may be formed of, for example, stainless steel andmay be shorter than a conventional stylus having, typically, a length of50 mm so as to minimise flexing.

Pivotal movement of the stylus 3 as it follows a workpiece surfaceduring use of the gauge is detected conventionally by using a linearvariable differential transducer (LVDT) 7. In the arrangement shown inFIG. 2, the coil 6 a of the LVDT is mounted in a conventional mannerwithin a recess provided in the main gauge body 1 while a ferrite core 6b of the LVDT is mounted, again in a conventional manner, to a shaft 14projecting from the end of the stylus 3. In this example, the shaft 14is formed from carbon fibre which is less affected by temperaturechanges than the materials previously used for forming the shaft andthus the dimensions of the shaft 14 and thus the location of the core 6b relative to the end of the stylus 3 do not alter significantly withchanges in temperature so that the output of the LVDT 7 is less affectedby temperature than would be the case if a conventional aluminum orsteel shaft 14 was used.

The stylus support assembly shown in FIGS. 2 to 4 is assembled by firstmounting the stylus 3 within the mounting block 2 and then coupling themounting block 2 to the main gauge body 1 by means of the resilientligament 4 and by clamping means 5 as discussed above. once the mountingblock 2 has been coupled to the main gauge body 1, the ferrite core 6 bneeds to be mounted to the shaft 14.

As will be appreciated from the above, when the mounting block 2 iscoupled to the main gauge support body 1, the stylus 3 is pushed into aposition at an angle to the longitudinal axis of the main gauge body 1.The resulting offset from the centre of the stylus 3 to the main gaugebody 1 is known from the angle of the angled front end face 1 b.

In order to accurately locate the ferrite core 6 b on the shaft 14, thestylus 3 and the main gauge body 1 are placed in respective conventionalV groove fixtures mounted so that one of the V groove fixtures ismovable along a micrometer screw thread relative to the other. Themovable fixture is moved through an offset distance which is determinedby the angle of the end face 1 b to the gauge axis so that the stylus isheld parallel to the longitudinal axis of the main gauge body. While thestylus 3 is held parallel to the longitudinal axis of the main gaugebody, the ferrite core 6 is adjusted on the shaft 14 until the LVDT 7provides a zero output reading. The ferrite core 6 b is then fixed inposition by, for example, cementing using medium viscositycyanoacrylate.

The stylus support assembly 13 is then removed from the fixtures. Thestylus returns to its angled position as it is no longer held parallelto the gauge body. A grub screw 8 located further towards the rear ofthe gauge body than the mounting block 2 is used to push the stylus 3back to a position where the transducer reading is again zero. The grubscrew is then released to allow the stylus tip to move by approximately0.2 to 0.25 mm. The range of movement of the stylus tip is thus at ±0.2to 0.25 mm which is within the linear range of the transducer. The upperend of the movement range is restricted by the recess 1 a.

The stylus tip 12 can be replaceable and can be changed dependent onwhat component is being measured. Thus, for example, the free end 3 a ofthe stylus 3 may be hollow so as to receive a shaft carrying the stylustip 12 which shaft may be secured in place by a suitable adhesive. Thetips can be manufactured from, for example, either synthetic ruby ortungsten carbide.

FIG. 5 shows an arrangement by which a stylus tip 12 may be correctlypositioned within the free end 3 a of the stylus 3.

The arrangement comprises a precision machined fixture 18 for accuratelylocating the stylus support assembly. A stylus tip alignment block 15 ismounted in a recess 18 a at one end of the fixture 18. The recess isadapted to receive a number of different alignment blocks 15 each havinga stylus tip receiving recess or datum face designed to accept andlocate a particular size or diameter of stylus tip. In order to alignthe stylus tip 12, the correct alignment block 15 is first positioned inthe recess 18 a and then the stylus support assembly, with the stylustip 12 loosely received on the free end 3 a of the stylus, is located inthe fixture so that the stylus tip is located against the alignmentdatum face.

The stylus tip 12 once aligned is then bonded in position using asuitable adhesive. A conventional thermo-softening adhesive such as atwo-part epoxy resin, may be used. This has the advantage of allowingthe stylus tip 12 to be removed and replaced by a different stylus tipsimply by heating the free end 3 a of the stylus 3 to soften theadhesive so as to allow manual removal of the stylus tip.

Although the gauge body and mounting block have been described ascircular cross-section members, it will be appreciated that they may beof any suitable cross-section, for example rectangular. The ligament andclamps would be correspondingly shaped.

A side acting LVDT transducer 7 is shown in the drawings. However, anysuitable transducer may be used with, for example, the transducer beingarranged to respond to up and down pivotal movement of the stylus. Also,instead of a turntable, either the stylus assembly or the workpiece maybe mounted to a slideway of conventional form to allow a stylus to betraversed across the workpiece surface to effect a measurement.

The stylus force providing arrangement described above is simpler toassemble than a conventional spring-biasing arrangement and is alsocheaper. In addition, the use of a single piece stylus and its securefixing within the mounting block prevents a user inadvertently removingor displacing the stylus. In addition, the materials used for the stylusand the ferrite core shaft 14 provide for increased thermal stability.

The above embodiment has been described by way of example and variousmodifications and alternatives will be apparent to those skilled in theart.

What is claimed is:
 1. A stylus support assembly for a stylus for agauge for a metrological instrument, comprising: a first member and asecond member for supporting a stylus, said first and second memberseach having first and second ends, with the first ends facing in thesame direction, and a resilient connection configured to connect thefirst ends, said first ends of the first and second members subtendingtherebetween an angle less than 180° so that the resilient connectionbiases the second member to extend at an angle to the first member.
 2. Astylus support assembly as claimed in claim 1 wherein a stylus is heldfixedly within the second member.
 3. A stylus support assembly. asclaimed in claim 2 wherein the length of the stylus is 50 mm.
 4. Astylus support assembly as claimed in claim 1 wherein the first memberhas a maximum width of 15 mm.
 5. A gauge for use in the metrologicalinstrument including a transducer and stylus support assembly as claimedin claim
 1. 6. A stylus support assembly for a metrological instrument,comprising: an elongate stylus having a first end for supporting astylus tip and a second end for supporting one of a core and a coil of atransducer, and an elongate member for supporting the other of the coreand the coil of the transducer, said stylus being secured to a mountingblock having an end face adjacent and facing in the same direction as anend face of the elongate member, said elongate member and the mountingblock being connected by a resilient plate extending over the end facesso that the stylus can pivot relative to the elongate member about apivot axis defined by the resilient plate, and said end face of theelongate member being at an angle to the end face of the mounting blockso that the resilient plate biases the stylus to extend at an angle tothe length of the elongate member.
 7. A stylus support assembly asclaimed in claim 6 wherein the stylus is formed in one piece having afront end and a rear end with a support for the transducer core at therear end.
 8. A stylus support assembly as claimed in claim 7 wherein themeans to support the transducer core is made of carbon fibre.
 9. Astylus support assembly as claimed in claim 7 wherein the front end ofthe stylus supports the stylus tip.
 10. A stylus support assembly asclaimed in claim 9 wherein the tip is made from synthetic ruby ortungsten carbide.
 11. A gauge for use in a metrological instrumentincluding a transducer and a stylus support assembly as claimed in claim6.
 12. A stylus support assembly for a stylus for a gauge for ametrological instrument, comprising: a first member and a second memberfor supporting a stylus, said first and second members each having firstand second ends, with the first ends facing in the same direction, and aresilient connection configured to connect the first ends, said firstends of the first and second members subtending therebetween an angle inthe range of from 177° to 178° so that the resilient connection biasesthe second member to extend at an angle to the first member.
 13. Astylus support assembly for a stylus for a gauge for a metrologicalinstrument, comprising: a first member and a second member forsupporting a stylus, said first and second members each having first andsecond ends, with the first ends facing in the same direction, and aresilient connection configured to connect the first ends, said firstends of the first and second members being at an angle to one another sothat the resilient connection biases the second member to extend at anangle to the first member, wherein adjustment means are provided withinthe first member to adjust a stylus position relative to a longitudinalaxis of the first member.
 14. A method of providing a predeterminedstylus force to a stylus for use in a gauge for a metrologicalinstrument, the stylus being supported by a stylus support assemblyincluding a first member arid a second member, said first and secondmembers each having first and second ends with the first ends facing inthe same direction, said method comprising the steps of: forming thefirst ends of the first and second members at an angle to each other,connecting the first and second members at the respective first ends bya resilient connection member, and forcing the second member to extendat an angle to the first member.